Galaxy formation: Too small to ignore

or more3, or equivalently can increase the eventual annual growth rate by about 0.06%. This may seem like considerable leverage. But the HDI cant go above 1, and many low-fertility countries already have HDI levels of around 0.93. Therefore, the best one can expect is an increase in TFR of 0.2, which is equivalent to raising the growth rate by about 0.4% from its currently projected lows. Countries such as Spain or Italy would still be below replace-ment, although, setting the social and political considerations to one side, they would be able to maintain their populations with far fewer immigrants.

Myrskyl and colleagues3 find important exceptions to the relationship between the TFR and HDI in some countries, including Japan, South Korea and Canada, the TFR continued to fall even when the HDI rose above 0.86. What might be happening here? The authors suggest that the positive effects of increasing HDI on womens decisions to have children may not apply in Asian countries because of social or cultural characteristics. Perhaps so, but what about Canada? These puzzling findings may instead be due to use of the HDI, which does not directly tell us which aspects of human development affect women rather than men. A different measure, the gender development index (GDI)4, describes the difference between male and female development. It would be use-ful to examine the relationship between the TFR and GDI, and to ask if countries such as Japan or Canada have a noticeable difference between trends in the HDI and GDI.

A final point worth stressing is that Myrskyl and colleagues3 also show that fertility in devel-oping countries, which have HDI levels much below 0.86, falls with increasing development. The social and environmental challenges of burgeoning populations in many developing countries, such as Bangladesh, Egypt, India and Pakistan, can only be addressed if these countries achieve and maintain low, below-replacement, fertility. Even in China, where low fertility was achieved by fiat, the maintenance of low fertility must eventually be driven by individual choice. In these and other develop-ing countries, increased human development, especially development that benefits women, is still the most powerful and most democratic route to achieving and maintaining lower populations. Shripad Tuljapurkar is in the Stanford Center for Population Research, and the Department of Biological Sciences, Stanford University, Stanford, California 94305-5020, USA.e-mail: tulja@stanford.edu

1. United Nations Population Division. Replacement Migration: Is it a Solution to Declining and Ageing Populations? (United Nations, 2002).

4. United Nations Development Programme. Statistics of the Human Development Report http://hdr.undp.org/en/statistics/indices (2008).

GALAXY FORMATION

Too small to ignoreKarl Glazebrook

A study of one galaxys dynamics backs up previous claims that surprisingly compact galaxies existed in the early Universe. But how such objects blew up in size to form present-day galaxies remains a puzzle.

Giant red elliptical galaxies are the oldest and most massive assemblies of stars in the nearby Universe. Large optical telescopes have tracked their evolution back through 11 billion years about 80% of the Universes lifetime by observing them at large cosmological red-shifts13. Observations seemed to indicate that nothing much had happened to them over this time, except that they grew rarer in the more distant past. It was thus a surprise when astronomers discovered recently that these galaxies have grown in size by a factor of five over this period while barely changing in mass. This is like suddenly discovering that Roman Londinium had the same population as Greater London does today. This extreme size and density evolution was not predicted by theories of galaxy formation, and remains difficult to explain.

One possibility is that measurements of the galaxies masses that are based on their lumi-nosities are flawed, not only because they might be missing starlight but also because they are not sensitive to the galaxies invisible dark-matter component (which does not contribute significantly to mass in the luminous regions, at least for nearby ellipticals). On page 717 of this issue, van Dokkum and colleagues4 report the first dynamical mass measurement which is sensitive to both the visible and the dark-matter component for an individual red compact galaxy, known as 12550, that is seen 10.7 billion years ago and is less than 1 kiloparsec (~3,000 light years) in size. This galaxy is about four times more massive and five to six times smaller than the Milky Way spiral. Distant red compact objects are widely considered to be ancestors of todays ellipticals owing to their similar stellar populations and morphologies. However, nearby ellipticals of similar mass have sizes of 310 kpc; none is of similar size and mass to 12550 (ref. 5).

A dynamical mass measurement is definitive but requires resolution of the internal velocity structure of the galaxy. If high-redshift ellipti-cals really are small but massive, it follows from Newtons law that their stars velocities should be very high. Because stars in ellipticals gener-ally move on eccentric orbits, this is measured using the stellar-velocity dispersion, which quantifies the average spread of velocities and can be determined from the subtle Doppler broadening of absorption lines in the galaxies spectra.

Measuring the velocity dispersion for 12550 was a tour de force4. Most of the light

of such a high-redshift galaxy is redshifted into the near-infrared waveband (12 m), where the airglow emission from the night sky con-tributes an enormous, noisy background. Van Dokkum et al.4 sustained a heroic 29-hour exposure time using the 8-metre Gemini South Telescopes Near-Infrared Spectrograph. This was just enough to detect the absorption lines in the galaxys spectrum a feat in itself for such a high-redshift galaxy and to measure the velocity dispersion. The measurement was partly helped by the large value of the final result: 510 km s1, the largest ever measured for any galaxy, but a value it had to have if it was as massive and as compact as previous measure-ments of its stellar mass had suggested6.

Van Dokkum and colleagues result4 is especially surprising because an earlier velocity-dispersion measurement7, based on a composite spectrum of several distant ellipti-cals (an interesting technique, albeit subject to its own problems), had measured a much lower value, sparking debate about the reliability of the stellar-mass measurements and the role of dark matter. The new measurement is sig-nificant because it is for a single object and so implies unequivocally that 12550 does have a density much higher than any nearby galaxy.

Cosmological time

BigBang

EarlyUniverse

3 billionyears

Present day,13.7 billion

years

Figure 1 | Bloating galaxies.As the Universe evolves with time, elliptical galaxies, which are believed to be very compact at early epochs, maintain much the same mass but become bigger, more diffuse and more abundant. Newly produced ellipticals somehow have similar properties to the evolving, older ones. Van Dokkum and colleagues spectroscopic analysis4 of a galaxy seen when the Universe was about three billion years old gives clear-cut evidence of the compactness and high density of such a high-redshift object.

So why are these compact, high-redshift galaxies such a theoretical conundrum? Ellip-tical galaxies have long been seen as the end-point of galaxy formation: when a star-forming spiral or irregularly-shaped galaxy, full of young blue stars, has its star formation quenched by some astrophysical-feedback process, it quickly ages to become a red and dead elliptical. As the overall star-formation rate of the Universe winds down with time, it seems natural to find an increasing number of these galaxy fossils full of old red stars. But such red galaxies are found to be more massive than any star-forming galaxy, so something extra is needed to provide the mass.

The consensus has been that elliptical gal-axies have also assembled through mergers of smaller galaxies, a process naturally expected in current galaxy-formation theories8. The most massive ellipticals would be the result of major mergers of smaller ellipticals with these progenitors having been of roughly equal mass. Elliptical galaxies are observed to follow tight scaling relationships between size, mass and velocity, which one might think would be seriously disturbed by mergers. However, computer simulations show that mergers sim-ply move the galaxies along the relationships without making them significantly less tight9.

But this picture breaks down when size evolution is taken into account: if you merge enough elliptical galaxies at high redshift to account for the size change, you also make many more high-mass galaxies than are observed in the nearby Universe. An alterna-tive is that if mergers are predominantly minor those in which a low-mass object merges into one of much larger mass size growth can be achieved without a substantial increase in mass10. However, low-mass galaxies gener-ally contain a lot of young stars, so this seems inconsistent with the observed old stellar pop-ulations of the high-redshift compact galaxies and their nearby descendants.

This lack of fit with the standard picture of elliptical-galaxy formation has driven a search for ways other than mergers by which the size of these galaxies could have blown up. For example, feedback processes such as an energy injection from a supernova11 or quasar12 could achieve that by expelling gas from the galaxy slowly (or rapidly), making the galaxys gravi-tational potential well shallower and moving stars into larger orbits. But these processes require a level of star or quasar activity that has not been observed. A more exotic explanation could involve the yet unknown nature of dark matter.

Any successful explanation of the size evolu-tion must solve what I call the synchronization problem, which in my view is the most fun-damental. The sizemass scaling relationship is tight in the nearby Universe, and possibly also at high redshift. It is just the normaliza-tion of this relationship that evolves. There are no massive compact elliptical galaxies today. Therefore, the high-redshift (early-epoch)

compact galaxies must be growing in size with time (Fig. 1). But, at the same time, the Uni-verse is making new elliptical galaxies, and somehow both the growing and the newly formed galaxies fall within the same tight sizemass relationship at all epochs. Their evolution is synchronized through some process that is either a coincidence or an important new piece of astrophysics.

A lot hinges on the interpretation of van Dokkum and colleagues single velocity-dis-persion measurement4 of 12550. As large telescopes acquire new multi-object, near-infrared spectrographs, we can expect to see many hundreds of such velocity-dispersion measurements in the next few years. We can also expect to see improved measurements of the structural and environmental properties of these compact galaxies, which will help us to figure out how bad the problems we have in explaining these objects really are. It remains to be seen whether we need conventional or

novel explanations for their astounding growth into the most massive elliptical galaxies we see today. Karl Glazebrook is at the Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia.e-mail: karl@astro.swin.edu.au

Music is a ubiquitous element in our daily lives, and was probably just as important to our early ancestors. Fragments of ancient flutes reveal that music was well established in Europe by about 40,000 years ago.

The Palaeolithic caves of the Swabian Jura in southwestern Germany have been a source of valuable and often provocative archaeological discoveries for many decades. In particular, finds of figurative art from the early Aurignacian the earliest Upper Palaeolithic archaeologi-cal culture associated with modern humans in Europe suggest that these huntergatherers had the knowledge, expertise, incentive and time to craft sophisticated objects for use in ritual activities. These activities probably served to affirm group affiliation, signal social identity and mark important social events or rites of passage. Conard et al.1 (page 737 of this issue) now reveal that the Aurignacian inhab-itants of the Swabian Jura had also mastered the art of music. Their detailed report high-lights the discovery of a largely complete flute (Fig. 1) and two small flute fragments in the

oldest Aurignacian layer at Hohle Fels Cave.Conard recently reported2 the discovery of a

female figurine carved from mammoth ivory in an Aurignacian layer at Hohle Fels dated to at least 35,000 years ago (based on the newly calibrated radiocarbon timescale). At present, this is the earliest such find in the world. Additional examples of figurative art of mammoths, horses, bison, cave lions, waterfowl and half-human, half-animal therianthropes have also been found in Aurignacian layers at Hohle Fels and other sites in the Swabian Jura. These finds suggest that the region was inhabited by a population of Homo sapiens sapiens that had mastered, among other things, the manipulation of mammoth ivory into three-dimensional, naturalistic forms for purposes not directly related to daily economic needs. Just as we continue to do today, these

Figure 1 | Sounds old. Conard et al.1 have discovered the oldest known flute, at Hohle Fels Cave in Germany. The flute is made from bird bone, and dates from the early Aurignacian, 40,000 years ago.